Variable electrical resistance structure



Dec, 11, 1956 A. J. MAsTRoPoLE VARIABLE ELECTRICAL RESISTANCE STRUCTURE2 Sheets-Sheet Filed Aug. lO, 1953 4L/#QED el MAS-reclame,

IN1/wrox rro/QA/E Dec. 11, 1956 A. .1. MASTROPOLE VARIABLE ELECTRICALRESISTANCE STRUCTURE 2 Sheets-Sheet 2 Filed Aug. l0, 1953 pm. e

E. L L @ff Am @o United States Patent() VARIABLE ELECTRICAL RESISTANCESTRUCTURE Alfred J. Mastropole, Allendale, N. J., assigner to G. M.Giannini & Co. Inc., Pasadena, Calif., a corporation of New YorkApplication August 10, 1953, Serial No. 373,331

19 Claims. (Cl. 201-59) This invention has to do with variableelectrical resistance and potentiometer structures of the general typein which a selected portion of the resistance winding is shiftable outof alignment with the body of the wind ing, and the brush electricallyengages the winding only at the deflected portion.

Potentiometers of that general type are described and claimed in thecopending patent application Serial No. 373,411, led on August 10, 1953,by Alfred J. Klose and entitled Potentiometer. The present applicationdescribes and claims certain features and improvements in such variableelectrical resistance structures.

More particularly, the features and improvements with which the presentapplication is concerned include means for preventing accidental damageto the resistance winding by movement of the brush beyond its intendedrange of operation, means for insuring the proper condition of tensionand position of the winding, and particularly convenient types of brushstructure.

A full understanding of the invention, and of its further objects andadvantages will be had from the following detailed description ofcertain illustrative embodiments, of which description the accompanyingdrawings form a part. That description is intended only as illustration,and it will be understood that many changes may be made in theparticulars of the described embodiments without departing from thescope of the invention, which is defined in the appended claims.

In the drawings:

Fig. l is a rear elevation of an illustrative potentiometer inaccordance with the invention, with cover removed;

Fig. 2 is a section on line 2 2 of Fig. l; K

Fig. 3 is a fragmentary section on line 3 3 of Fig. l at enlarged scaleand showing the brush rotated into the plane of the section;

Fig. 4 is a fragmentary section on line 4 4 of Fig. 3;

Fig. 5 is a fragmentary section on line 5 5 of Fig. 2;

Fig. 6 is a section corresponding to Fig. 2 and showing an illustrativemodification;

Fig. 7 is a fragmentary portion of Fig. 6 at enlarged scale;

Fig. 8 is a fragmentary section, corresponding to a portion of Fig. 3and showing an illustrative modification; and

Fig. 9 is a fragmentary section on line 9 9 of Fig. 8.

Figs. l-5 represent an illustrative embodiment of the invention. Asupporting frame member, indicated generally at 20, includes acylindrical sleeve portion 22 with an external tlange 24 at its forwardend. An axial through bore 26 in frame member 2l) provides convenientmounting for the ball bearings 28 and 29 by which the control shaft 30is journaled. Forward bearing 28 is preferably protected, as by theshield 27, which is retained by the plate 31 and dust seal 32. Threadedmounting holes 33 may be provided in the forward face of frame flange24, that ange portion of the frame then constituting a mounting plate.

A resistance winding is carried on a drum 40 of di- 2,773,966 PatentedDec. 11, 1956 ICC electric material, which comprises a cylindrical bodysection 42 and a drum flange 44. An axial bore in the drum receivessleeve portion 22 of the frame member, and the frame and drum aresecured rigidly `together by the nuts 45 which turn on threaded rods 46.The latter are threaded into the rear face of frame flange 24 and arereceived by holes in the drum ange. A protective cover for the drum andwinding is shown as a light sleeve 50, the forward end of which fitsover the periphery of drum ange 44 and abuts frame flange 24, and a backcover plate 52, which closes the rear end of sleeve 50 and is secured bythe screws 53. As shown, those screws are internally threaded and gripthe ends of rods 46.

The resistance element, indicated at 60, may be of any type that issomewhat flexible, such, for example, as a single resistance wire or anelongated and relatively finely wound coil of such wire. Resistanceelement 60 is helically wound on the outer cylindrical face of drum body42. That face is preferably provided with a helical channel, which maycomprise the threads 62, to receive and locate the several turns of thewinding in axially spaced relation, so that electrical contact betweenadjacent turns is positively prevented without requiring more than arelatively slight physical spacing between them. As a typical example,which is not intended in any sense to limit the scope of the invention,the resistance wire may be only a few thousandths of an inch indiameter, and the threads 62 in which it is received may, for example,have a pitch corresponding to as many as 72 threads per inch. A greatvariety of thread forms may be used successfully, a thread having anincluded angle of about 60 to 90 being preferred. The threads preferablycover a portion of the drum surface of greater axial extent than thewinding itself. Since it is sufficient that the winding be supported ina surface that approximates a figure of revolution about the drum axis,drum body 42 need not necessarily have a continuous outer face. Anillustrative type of brush assembly in accordance with the invention isindicated at 80. A brush-supporting rotor plate 82 of insulatingmaterial is lixedly mounted on the rearward end of control shaft 30 inany convenient manner, as by the mounting nuts 83 and 84, which arethreaded directly on the shaft and clamp the rotor plate between them.The brush proper is shown at 85, and a pick-up contact shoe at 86,fabricated typically from a single piece 87 of resilient sheet materialsuch as a resilient bronze alloy. The central portion of contact piece87 is tixedly mounted on rotor plate 82, as by the screws 83. Pick-upshoe 86 extends from that point of connection radially inward across thecylindrical axis of the drum, and yieldingly engages the inner end of acontact pin 90, which is supported on the axis of the instrument as bythe grommet 91 in insulated relation to cover plate 52. The outer end ofpin 90 constitutes an external terminal post for making electricalconnection to the potentiometer brush.

The brush proper extends radially outward from the point of connectionat 88, and is bent forwardly to form a substantially straight contactarm extending approximately axially in spaced relatie-n to the outercylindrical surface of drum body 42. The arm is so formed that whenunder no constraint it extends obliquely outward, as indicated inphantom lines at 85a. One turn of winding 60 extends tangentiallyoutward from the drum threads 62, as indicated at 61, and slidinglyengages the radially outer face of brush 85, as indicated at 63. Thewire is held yieldingly taut by the outward spring of the brush, and inturn deects the brush radially inward from its natural position 85a tosuch a position as that shown in solid lines at 85 in the drawings. Theaxial position at which the lifted turn 61 contacts the brush is notdefined byy the structure of the brush itself, which in the presentembodiment presents a smoothly polished outer surface to the wire, butis determined rather by the action of threads 62 of the drum in guidingthe wire at each side of the brush.

The shape of the brush proper has been found to be of basic importanceto insure long life and reliable action, particularly when theresistance wire is of small diameter, for example of the order of onemil. As shown inFig. 3, the cross section of the brush arm is preferablybowed outwardly with a radius approximately equal to 0.4 times theradius of the resistance winding and about twice the width of the arm.The corners of the brush, `over which the winding slides, are slightlybroken, and theentire Working surface of the brush is carefullypolished.l

totation of brush S5 with respect to the winding support 42 ispositively limited to a predeterminedrange of movement by a positivestop mechanism shown illustratively at 100. The rotation of shaft 30with .respect to frame member 20 is utilized-to produce axial movementof two nuts, which arein threaded-relation'with one of those members andin splined relation withfthe'other. As illustratively shown, the nuts102 and 103 engage the threads .101, provided on shaft'lti'between thebearings 28 and 29. The pitch of threads 101' in the present ernbodimentneed not be the same as that of winding channel 62. Rotational movementof the nuts withrespect to the frame member is prevented in any suitablemanner, as by the radial pins 104 and 105, fixed in the periphery of therespective nuts and slidingly received in the axial slot 106 in the wallof bore 26. Accordingly, as the shaft is turned, the nuts move togetheraxially along the shaft without rotation relative to the splined member,here taken as the frame.

Stop formations 110 and M1 are mounted in the nonadjacent axial faces ofthe respective nuts 102 and 103 at definite rotational positions withrespect to spline formations 104and 105; and cooperating stop pins 108and 109 are set in the member with respect to which the nuts 'arethreaded, here shown as shaft 30, in position to engage the respectivestop formations in response to shaft rotation. The parts are soproportioned that as the shaft rotates in one direction the stop pintoward which the nuts are thereby moved clears the stop formation on thenearer of the two nuts by a comfortable margin on one rotation andpositively engages that formation on the next following rotation. Bysuitable angular and axial spacing of the pins, any desired integral orfractional number of rotations of the shaft may be permitted between thepoints at which the shaftis positively stopped, that number ordinarilycorresponding to the number of available turns of the resistive windingover which the brush can effectively travel.

A particular advantage of the type of stop mechanism illustrated is theconvenience and economy with which it can be adapted for resistancewindings of different lengths. Such adaptation of the stop mechanism canbe made duringassembly, and requires no modification,ofstructural forrnof the individual parts. For example, by assembling the nuts 102 and 103on shaft 30 farther apart by one thread than the positions shown in Fig.2, the range of rotational movement to which brush 85 is confined isreduced by one full revolution. Similarly, that range may be increasedor decreased by any integral number of revolutions within the capacityof the structure by suitable relative axial spacing ofthe two nuts ontheir threads. Further modification of the range of movementmay beprovided by varying the relative rotational positions of the severalcooperating elements of the mechanism. For example, a plurality of holes107, shown in Fig. 5 as six, may be provided in the upper face of thenut 103 in different rotational relations to spline pin 10S; and duringassembly a stud may be mounted in a selected one of those holes toproduce .the stop formation 111. trative manner, the` range of movementof .brush-85 may ln that illusl be varied conveniently from oneinstrument to the next by a fractional number of revolutions.

The ends of winding 60 are anchored with respect to the supporting drum42 at points spaced beyond the respective ends of the range of movementto which brush 85 is limited by stop mechanism 100. lust enough wire isallowed between the points of extreme travel of the brush and theeffective points of anchorage of the wire to the drum to permit normaland uniform interaction of the brush and wire over the Whole range ofbrush travel. Preferred anchoring means, as illustratively shown,coniprise a slotted stud 65, set firmly but rotatably in drum 42. Thehead of stud 65 is slotted and is preferably positioned ywithin a recess66 and below the level of threads 62. The winding end is secured to stud65 in any suitable manner, for example by solder, as indicated at 67.After the winding has been thus anchored, its length can be alteredconveniently by turning stud 65 in its radial bore, as by a screwdriverapplied to its slotted head. The tension of the winding is therebyadjustable much in the manner of a violin string.

Channels may be formed through the body of the drum, as indicated at 68and 69, through which connection may be made between the ends of thewinding and respectivel binding posts set in the rear face of thedielectric drum flange at 77 and 7S. In Figs. 3 and 4 axial channel 69is shown with a connecting radial bore 73 which opens into the recess 66at the forward end of the winding, and gives access for connection tostud 65 to which that winding end is anchored at 67'. A wire'ofrelatively low resistance is indicated at 74 leading from that studthrough channels 73 and 69 and radially outward through a secondconnecting radial bore 75 closely adjacent the rearward wall of drumfiange 44. A similar connection 76 from the rearward end of winding 60passes through channel 68 via suitably placed radial bores. Wires 74 and76 are connected to the respective terminal posts 77 and 78, which arethreaded into the drum flange and are guided in insulation relationthrough cover plate 52 as by the rubber grommets 79, and which provideexternal terminal posts for making electrical connection to theresistance element of the potentiometer.

In assembling the winding, it is found convenient first to connect therearward end of the winding to its stud 65 and then to start Windingfrom the rear forward, the brush. being already assembled but free ofthe winding.. The drum is slowly rotated, with the brush heldstationary, so that the brush in effect follows the wire as itfalls-.into place in the threads of the drum. The last Aturn of theWinding is passed over the brush and then connected as at 67 to its stud65, as by solder. Before that connection, the tension of the winding isadjustedroughly to draw the brush nearly into' parallel relation with.the vouter cylindrical face of the drum. Final adjustment of the'tension of the winding, preferablyV bringing the brush accurately intoaxial alignment with the drum surface, may then be made by turning thestudsS, in the manner already described.

MeansV are preferably provided for convenient and accurate adjustment ofthe angular position of the brush on control shaft 30. 4A `particular'advanatage of such adjustment is yto facilitate productionof the desiredrelation between the'endcpointsof the brush-movement, as :limited `byvstop mechanism 100,` andA the anchored ends 'ofwinding 60. 'Thewindingends are preferably anchored 4at points spaced angularly (asmeasuredalong the Windinglby more than the angle through which the brushis allowed to turn by stop'mechanism 100, the excesscorrespondingsubstantially to the angular length of the portion61 of thewinding that is supported by thebrushin spaced relation to mounting drum42. The brush is then preferably-.so adjusted on its shaft that' itsaverages angular range of movement is centered within the free portionof winding 60 between its anchored ends.

In the present embodiment, such adjustment is provided by mounting brushsupport 82 on shaft 30 by frictional means that lock the rotationalposition of the brush on the shaft effectively positively during allnormal operation of the potentiometer, and yet permit their manualrelative yrotation when adjustment is required, as during assembly. Forexample, the shaft may be rotated to one end of its range of movement,coming to a definite position determined by stop mechanism 14MB, andbrush holder 30 may then be turned manually against its frictionalmounting on the shaft to bring the brush to the desired distance fromthe winding anchor point 67. As shown, such adjustment is made possibleby interposing a spring washer 89 between brush support 82 and uppermounting nut 84. Before mounting the brush assembly, lower mountingwasher 83 is adjusted on the shaft to retain bearing 29 in position toreduce shaft end play to a minimum. Nut 84 is turned down over brushsupport 82 to compress washer 89 appreciably, but not to its solidthickness, producing suiiicient friction to make brush holder 82effectively rigid on the shaft during normal operation, but readilymovable manually for adjustments of the type described.

In the embodiment of Figs. l-5, brush 85 moves in a circular path aboutthe axis of the winding. Because of the helical form of the winding, thecontact point at 63 between winding and brush progress longitudinally ofthe brush in an axial direction. Alternatively, the brush may be causedto move in a helical path, so that it progresses axially with a pitchequal to that of the winding. ln Figs. 6 and 7 is shown an illustrativeembodiment providing that type of action, and in which axial movement ofthe brush is coordinated with the axial movement of the stop mechanism.

As illustratively shown in Fig. 6, Shaft 110 is journaled near itsforward end in frame 100, and is threaded at 112 to receive the singlestop nut 120 at the rear of the journal bearings 116 and 117. Two stopnuts may be used if preferred, as already described. Nut 120 carries thestop formations 121 and 122 on its respective axial faces and the splinepin 124, which is slidingly received in axial channel 126 of framemember 100. Stop pins 128 and 129 are fixed in the shaft and cooperatewith the stop formations of the nut, in a manner already explained, tolimit relative rotation of the shaft and frame to a predetermined range.

The brush carrier 130 of Fig. 6 has a sleeve portion 131 that is axiallyslidable on the crests of the threads 112 and is caused to rotate withthe shaft, as by the axial slot 132 which receives stop pin 129. Acompression spring 136 acts between the brush carrier and a disk 138fixed on the rearward end of shaft 110. That spring urges the forwardend of brush carrier sleeve 121 yieldingly against nut 120, causing thebrush carrier to move axially with the nut.

The brush, as shown, comprises a resilient strip 140, one end of whichis rigidly mounted in insulated relation on brush carrier 130 and whichcarries at its free end a contact formation 141. Electrical connectionfrom the brush may be made via a light flexible wire 143 to a contactshoe 145 mounted in insulated relation in disk 138 and resilientlyengaging the inner end of a fixed terminal post 146 on the axis ofrotation of the shaft. Brush carrier 139 and disk 138 are preferably ofinsulating material.

The resistance winding 159 is carried in a thread-like helical channel152 formed in the outer cylindrical surface of dielectric drum 154. Thelatter is mounted on frame 1S@ and forms with it a winding support ofcylindrical shell formation. Winding channel 152 in the present instanceIhas a helical pitch substantially equal to the pitch of shaft threads112. One turn of Winding 150 passes slidingly over contact formation 141of the brush at 151 and is supported in spaced relation to the drumsurface.

Rotation of shaft causes brush 140 to rotate with it, lifting theresistance winding progressively from its channel and permitting it todrop back into that channel after passage of the brush. At the sametime, the brush is caused to move axially with nut 120, with the resultthat contact formation 141 follows the helical form of the winding.Contact is therefore always made at substantially the same point of thebrush. The ends of the winding may be anchored in any suitable manner,for example as previously described.

A particularly advantageous feature of the embodiment of Fig. 6 is thedual function of nut 120. It will be noted that travel rof nut performsthe two functions of actuating the stop mechanism, in a manner analagousto nut-s 102 and 103 of the previously de scribed embodiment, and `ofdriving the axial movement of brush 140, a function not present in theprevious embodiment.

Figs. 8 and 9 represent an illustrative type of brush structure, whichcan be employed, for example, in connection with the overallconstruction of either of the previously described embodiments, butwhich is here described typically as a modification of the embodiment4of Figs. l-5. As in those figures, winding 60 is carried in helicalchannel 62 in the convex cylindrical surface of drum 42, and brushcarrier 82 is mounted lon a shaft journaled on the axis of that drum. Inthe present modification the brush proper comprises a structure adaptedto slide in, and be guided by, direct contact with helical channel 62and to support the winding in spaced relation with the channel atsubstantially a single point. Structure 160, as shown, is formed from asingle piece of sheet metal bent in U-form, with the bend of the Usuliiciently pointed in section to form a V-edge 162 that will tit andride positively in the thread channel 62. Each leg of the U is formedwith two iiat shoulders 164 with a projecting tongue 165 between them,all four shoulders 164 lying in a common plane parallel to edge 162, andthe two tongues being aligned substantially at right angles to thatedge, allowance being preferably made for the helix angle of threads 62.The V-edge 162 is preferably curved to conform to the curvature of thosethreads, as shown best in Fig. 9. The interior of the U, opposite edge162, is preferably more smoothly curved, as indicated at 163, providinga suitable surface over which the resistance wire may slide as thecontact member moves under it.

The movement of brush 160 longitudinally of helical channel 62 iscontrolled by an arm 170, which is shown of fiat and somewhat resilientmaterial having one end secured rigidly to brush carrier 82 andextending parallel to the axis of the instrument closely spaced radially:outward of the winding. A longitudinal slot 171 in arm freely receivesthe two tongues 165 of the brush proper, and the inner face of the armrests flatly on the shoulders 164. Arm 170 is so formed as to normallylie closer to threads 62 than shown in Figs. 8 and 9, so that it urgesbrush 170 yieldingly into firm but light engagement with those threads.'lio accommodate the axial movement of brush 170, and provide reliableelectrical Contact, a flexible connection 172 is preferably providedbetween the brush proper and arm 170. The arm may be connected viascrews 173 and a contact shoe 86a to an external terminal similar to 90of Fig. 2, for example.

An advantage 'of the described structure is that it permits reduction toa very low value the distance that resistance winding 60 must be liftedfrom its normal position in channel 62. Particularly in view of thatrelatively short distance, the inherent resiliency of the resistancewire is sutiicient for effective operation, and

resilient structure in the brush itself is ordinarily not necessary.

I claim:

l. A variable electrical resistance structure, comprising a cylindricalshell, a resistance winding carried in insulated relation on the convexsurface of the shell, a mounting plate at the forward end of the shelland normal to the axis thereof, a control shaft journaled on the axis ofthe shell and operable from forward of the mounting plate, brush meansmounted `on the shaft and extending therefrom at the rear of the shellincluding a substantially linear contact formation extending forwardlysubstantially parallel to the convex surface of the shell and closelyspaced radially outwardly thereof, a portion only lof the winding being-supported by the contact formation in spaced relation with the shell,said shaft having a thread within said shell, a nut engaging saidthread, said nut being slidingly keyed with respect to said shell, and astop formation arranged to engage said nut to limit the' rotation ofsaid shaft to a predetermined limited range that includes more than onerotation.v

2. A variable electrical resistance'structure as defined in claim l, andin which the brush means includes a brush support slidingly mounted onthe control shaft for axial movement thereof and slidingly keyedthereto, and resilient means urging the brush support toward the nut.

3. A variable electrical resistance structure, comprising a generallycylindrical support, a brush mounted for relative rotation with respectto the support about the axis thereof and spaced radially outward of theconvex surface of the support, and a .resistance winding supported onthe convex surface of the support, one turn of the winding passingradially outward of the brush t in electrical contact therewith, andmeans for adjustably varying the tension of the winding, said meanscomprising a slotted stud in said support secured to said winding at oneend.

4. A variable electrical resistance structure, comprising a generallycylindrical support, a brush mounted for relative rotation with respectto the support about the axis thereof and radially outward of the convexsurface of the. support, stops means acting between the brush and thesupport to limit movement of the brush with respect to Athe support to apredetermined range that includes more than one complete relativerotation, said stop means including a pair of engaged helical threadelements, one of said elements being coupled to said brush to preventrotation thereof relative to the brush and the other being coupled to-said support to prevent rotation thereof relative to the support, and aresistance wire supported on the convex surface of the support, aportion only of the wire passing radially outward of the brush inelectrical contact therewith, the wire being anchored at its ends withrespect to the support at points spaced beyond the ends of the saidrange of movement of the brush with respect to the support.

5. A variable electrical resistance structure, comprising: a cylindricalsupport member, a control shaft member journaled on the axis of thesupport, threaded means coaxial with the shaft within the support, thesaid means being slidingly keyed with respect to one of those membersand in threaded relation to the other, and stop formations`r carried Vbythe said means and by the said other member, respectively, andcooperating to limit relative rotation of the shaft and the support to apredetermined limited range that includes-more than one revolution,brush means mounted for rotation with the control shaft and including acontact formation closely adjacent the convex surface of the support, aresistance winding carried in insulated relation on the convex surfaceof the support, a portion only of the winding being supported by thecontact formation in spaced relationr to the support, the windingbeinganchoredwith respect to the support at points-.spaced beyond the ends ofthe range of movement-of the contact formation.

6. A variable electrical resistance structure, comprising a generallycylindrical support, a control shaft journaled on the axis of thesupport, two threaded elements coaxial with the shaft within thesupport, the said elements-being slidingly keyed to one of those membersand in threaded relation to the other, the elements being 4spacedaxially rof the threads, a stop formation carriedvby each of theelements, two stop formations carried by 'the said other member andcooperating with the respective first mentioned stop formations to limitrelative rotation of the shaft and the support to a predeterminedlimited range that is variable by variation of vthe said axial spacingof the two threaded elements, brush means mounted for rotation with thecontrol shaft and including a contact formation closely adjacent thesurface of the support, a portion only of the winding being supported bythe contact formation in spaced relation to the support, the windingbeing anchored with respect to the support at points spaced beyond theends of the lrange of movement of the contact member.

7. A variable electr-ical resistance structure, comprising a cylindricalsupport, a control shaft journaled on the axis yof the support forrelative rotation with respect thereto, a nut coaxial with the shaftwithin the support, the nut being slidingly keyed with respect to one ofthose members and in threaded relation to the other, stop formationscarried by the nut and by the said other member, respectively, andcooperating to limit relative rotation of the shaft and the support to apredetermined range, brush means mounted for rotational movementwith't'he shaft and for axial movement with the nut and including' acontact member movable by virtue of shaft rotation along a helical pathof limited length closely spaced radially outward with respect to theconvex surface of the support, the length of the said path being limitedby virtue of the said limited range of relative rotation'of the shaftand the support, a resistance winding helically wound in insulatedrelation on the convex surface of the support with a pitch substantiallyequal to the pitch' of thev threads between the nut and the said othermember, a portion only of the winding electrically contacting thecontact member and being supported thereby in spaced relation to thesupport, and means anchoring the winding at points spaced beyond therespective limits of the said helical path of movement of the contactmember.

8. A variable electrical resistance structure, comprising a cylindricalshell, a control shaft journaled on the axis 'of the shell, a nut inthreaded relation to the shaft and slidingly keyed to the shell, a brushcarrier axially movably mounted on the shaft for rotation therewith,means causing the brush carrier to move axially of the shaft withthenut, stop formations carried by the nut and by the shaft,respectively, and cooperating to limit the relative rotation of theshaft and the shell to a range that includes more than one fullrevolution, a brush carried by the brush carrier for axial androtational movement therewith in a helical path closely adjacent theconvex surface of the shell through a limited range that corresponds tothe said range of relative rotation of the shaft with respect to theshell, a resistance winding helically woundy in insulated relation onthe convex surface of the shell with a pitch substantially equal to thepitch of the threads between the nut and the shaft, a portion only ofthe winding electrically contacting the brush and being supportedthereby in spaced relation with the shell, and means anchoring thewinding at points spaced beyond the respective ends of the said range ofmovement of the brush.

9. A variable electrical resistance structure, comprising a cylindricalsupport having a generally helical channel in its convex surface, thewalls of the channel being of I dielectric` material, a resistancewinding normally lying freely in the channel, a contact shoe slidable inthe channel, control means for the contact shoe comprising a memberrelatively rotatable with respect to the cylindrical support about theaxis of the support said member being in radially spaced relation to theconvex surface of the support and slidingly engaging the contact shoe,the contact shoe supporting a portion only of the resistance winding inspaced relation to the channel walls and electrically contacting theresistance winding within the said portion thereof.

10. A variable electrical resistance structure as defined in claim 9 andincluding a slotted stop formation projecting from the convex surface-of the support in position to limit movement of the shoe longitudinallyof the channel, the resistance winding being received by the slot in thestop formation.

Y 1l. A varia-ble electrical resistance Structure, comprising acylindrical support having a generally helical channel in its convexsurface, the walls of lthe channel being of dielectric material, a shoeriding in the channel and having a portion extending radially Ifrom thebody of the shoe, a shaft journaled 'on the axis of the support, acontrol member mounted on the shaft for rotation therewith and spacedradially from the convex surface of the support, the control memberhaving an 'axially extending slot in which the radially extendingportion of the shoe is slidingly received, a resistance wire vnormallylying freely in the channel, a portion only `of the resistance windingbeing electrically engaged by the shoe and supported thereby adjacentthe point Iof engagement in spaced relation fto the channel walls.

12. A variable electrical resistance structure, comprising a cylindricalsupport, a control shaft journaled on the axis `of the support, stopmeans acting between the control shaft and the support to limit theirrelative rotation to a definite predetermined yangular range, aresistance wire helically wound on the convex surface of the supportwith its ends anchored thereto at points that are mutually spaced bymore than the said angular range, brush means electrically contactingIand supporting a portion only of the wire in spaced relation to thesupport, and means mounting the brush on the shaft for rotationtherewith through a limited angular range corresponding to the saidangular range of the shaft, the said mounting means normally definingthe rotational position of the `brush on the shaft eectively rigidly andpermitting manual relative rotational movement of the brush and shaft tocenter the angular range of movement of the brush between the anchoredends of the winding.

13. A variable electrical resistance lstruct-ure comprising a frame, acontrol shaft journaled on said frame and rotatable abou-t an axis,convex cylindrical support means on said frame and concentric with saidaxis, a brush means on said frame, one of said means being xed to saidframe, the yother of said means being revolvable about said axis andbeing coupled to 'said shaft to be revolved thereby relative Ito said`one means, a pair of thread elements in engagement with each other andconcentric about said shaft, one of said thread elements being movablein the direction o-f the length of lsaid axis, one of said lthreadelements being coupled to said shaft -to be revolved thereby about saidaxis and the other being coupled to said frame to prevent itsrevolution, whereby said movable thread element is moved 'axially `assaid shaft is rotated, a pair of cooperating stop formations to limitthe relative revolution of said means to a predetermined limited rangethat includes more than one revolution, one of said stop formationsbeing axially movable and connected to said axially movable threadelement, one Iof said stop formations being coupled to said shaft to berevolved thereby about said axis and the other being coupled to saidframe to prevent i-ts revolution, said brush means including a contactformation closely adjacent the convex support means, a resistancewinding carried in insulating relation on said convex support means, aportion of the winding being supported by the contact formation inspaced relation to the support, the winding being anchored to saidconvex support means at points spaced therearound beyond the ends ofsaid limited range of relative revolution.

14. A variable electrical resistance structure as claimed in claim 13and in which said convex cylindrical 'support means defines a generallyhelical channel of uniform cross section having walls fof insulatedmaterial, said resistance winding normally lying freely in said channel,said Contact formation having an extremity shaped to fit in saidchannel, the pitch of said channel being substantially equal to thepitch of said thread elements.

15. A variable electrical resistance structure comprising a frame, acontrol -shaft j'ournaled on said frame and rotatable about an axis,convex cylindrical support means on said frame and concentric with saidaxis, a brush means `on said frame, one of said means being fixed tosaid frame, the other of -said means being revolvable about said axis`and |being coupled to said shaft to be revolved thereby relative tolsaid one means, at least a portion `of said shaft being threaded, athreaded element slidingly keyed to said frame lfor movement axially ofsaid shaft 4and engaging the threaded shaft portion, a stop formationfixed to the shaf-t arranged to engage said threaded element to limitrelative revolution of said means to a predetermined limited range that-includes more than yone relative revolution, and a resistance windingcarried 4in insulating relation on said con-Vex support means, a portionof the winding -being engaged by said brush means, the winding 'beinganchored to said convex support means `at points more widely spaced thanthe ends of said range of relative revolution.

16. A variable electrical resistance structure as claimed in claim 15and ,wherein said Ithreaded element is a nut on said shaft, said nuthaving a plurality `of stop holding positions, and a stop removablysecured to said nut at one Aof said positions, said stop yformationbeing arranged to engage said stop.

17. In a variable electrical resistance structure of the multi-turn typehaving a helical resistance element with a plurality of turns carried ininsulated relationship on a supporting drum and a contact elementengaging said resistance element, said resistance and contact elementsbeing revolvable with respect to each other, and a drive shaft arrangedto revolve said resistance and contact elements with respect to eachother: positive-acting precise stop mechanism enabling said resistanceand contact elements to revolve with respect to each other for more thanone complete turn and yet positively and precisely stopping saidrevolution at a predetermined position comprising a rotatable feedscrewdriven by said shaft in predetermined relationship with respect to therevolution of the resistance and contact elements with respect to eachother, a travelling member engaging said feedscrew, said travellingmember including means cooperating with said supporting drum to preventrotation of said member on said feedscrew while allowing translation ofsaid member along said feedscrew as said feedscrew rotates, a first stopelement on said member and a second stop element driven by said shaftand revolvable with respect to said iirst stop element, said first andsecond stop elements missing each other during one turn of saidfeedscrew and said second stop element positively abutting against saidfirst stop element during a subsequent turn of said feedscrew.

18. A variable electrical resistance structure comprising a generallycylindrical support having a convex surface with a helical channeltherein, a brush mounted for relative rotation with respect to thesupport about the axis thereof and spaced radially outward of the convexsurface of the support, a resistance winding supported on the convexsurface of the support in said helical channel, one turn of the windingpassing radially outward of the brush in electrical contact therewith,and means for adjust'- ably varying the tension of the windingcomprising a rotatable lstud` secured to, saidw'indingat oneendend-positioned within` a recess in said support below therlevel ofsaid helical channel, and afterminal postelectrically connected to saidrotatable stud; l

19. A variable electrical resistance structure compris.- ing a frame, acontrol shaft journaled onv said frame and rotatable about yan axis,convex cylindrical support means on said frame and concentric with saidaxis,l a brush means on said frame, one of said means being fixed tosaid frame,. and the other of said means being revolvable about saidaxis and being coupledto said shaft to be revolved thereby relative `toysaid one means, apart on said shaft being threaded,- said support meanshaving `a recess therein extending longitudinally thereofa threadedelement engagingY the threaded part on the vshaft and having aprojectingportion eiectively slidingly keyed in'to said recess-in thesupport means for movement axially of said shaft While preventingrotation of said threaded element with respect to said support means, apair'of spaced stop formations fixed vto the shaft on eithersidevof saidthreaded element, said threaded element travellingnaxially with respectto said shaft betweenl said stop formations and arranged to engage saidstop formations at leach end of its travel to limit relative revolutionof said means to a predetermined limited range'that includes more-thanone relative revolution, and a resistance winding carried in insulatingrelation on said convex support means', a portion of the winding beingengaged by said brush means.

References Cited in the-le of this patent UNITED STATES PATENTS`1,918,013 v Bermann July 11, 1933 2,442,469 Palya June 1, 19482,495,321 Gibbs et al. Jan. 24, 1950 2,519,752 Fox` -Aug. 22,19502,533,656 Wills Dec. 12, 1950 2,662,150 Mars Dec. 8, 1953

